WASHINGTON (AP) — Secretary of State John Kerry is warning Russia that it will face an immediate, “very serious series” of steps from the United States and Europe if it annexes Ukraine’s strategic Crimea region.

Kerry told a Senate committee that Moscow should expect the U.S. and European Union to take measures against it on Monday should it accept and act on the expected results of this weekend’s referendum in Crimea. Crimeans are to vote on joining Russia on Sunday in a referendum that the U.S. and EU say violates Ukraine’s constitution and international law. Russia has said it will respect the results of the referendum.

“There will be a response of some kind to the referendum itself,” Kerry told the lawmakers on Thursday. “In addition, if there is no sign (of Russian compromise) there will be a very serious series of steps on Monday in Europe and here.”

“My hope is they will become aware of the fact that the international community is really strongly united,” he said.

Kerry was to leave Washington later in the day to meet Friday in London with Russian Foreign Minister Sergey Lavrov in a last-ditch bid to avert a crisis over the referendum.

He said he had spoken to Lavrov before the hearing and that hoped “reason would prevail.” But stressed there was no guarantee of that.

Kerry and Lavrov have spoken almost daily as the Ukraine crisis has unfolded but have yet to find any common ground.

He suggested that he would be pressing Lavrov for Russia to accept “something short of a full annexation” of Crimea but did not elaborate on what such a scenario might entail.